1. Field of the Invention
The present invention relates generally to scanners and, more particularly, to multiple resolution scanners.
2. Description of the Related Art
Optical scanners are found in a variety of devices including, but not limited to, stand alone scanning devices, facsimile machines, copiers and combination scanner-copier-printer-facsimile machines. The optical scanners capture and digitize images of, for example, printed matter such as text and photographs on sheets of paper or other substrates. The digitized images can be printed, transmitted to a remote location, or electronically stored for later printing, transmission, processing or manipulation.
Conventional optical scanners include a charge coupled device (CCD) sensor or other linear array of photoelectric sensing elements, a light source, an analog amplifier, an analog-to-digital converter, a controller and a random access memory. CCD sensors typically include a large number of photoelectric sensing elements, each capturing light representing a single pixel of the image. The sensing elements, which are arranged in a linear array, will capture a line of pixels. As such, an object can be scanned one line at a time by moving the array relative to the object (in, for example, a flat bed scanner) or by moving the object relative to the array (in, for example, a sheet fed scanner). The light source is typically a lamp, such as a cold cathode lamp or a xenon lamp. Light from the source that is either reflected from an object, or transmitted through the object, is focused onto the CCD sensor with a mirror and lens arrangement. Each photoelectric sensing element converts the light that it receives into an electric charge, the magnitude of which depends on the intensity of the light and the exposure time. The charges are converted into analog voltages via the analog amplifier and the analog voltages are then digitized by the analog-to-digital converter.
Some optical scanners are capable of operating in two or more resolution modes. The resolution level is varied by varying the size of the object that is projected onto the sensing elements without varying the number of sensing elements that are being used. The smaller the object, the higher the resolution. The resolution level is changed through the use of interchangeable lenses. For example, one lens may cause an 8½ inch object to be projected onto the sensing element, while another may reduce the size of the object to 4 inches by focusing the middle portion of the text or image bearing substrate onto the same sensing element.
One problem associated with conventional optical scanners is the reduction in image quality that is caused by variations in the intensity of the light emitted by the lamp. Such variations occur while the lamp is warming up to its operating temperature, where the light intensity is stable. Cold cathode lamps, for example, can take up to 60 seconds or longer to warm up. Light intensity at the operating temperature will also vary over time as the lamp ages. This problem is especially acute in color scanners where the intensities of red, green and blue change with respect to one another when the light intensity changes, which leads to hue errors. Thus, despite the fact that optical scanners typically performed “white reference” and “black reference” calibrations at start up, variations in light intensity during scanning operations were problematic.
A conventional solution to this problem was to simply delay the scanning process for a predetermined period sufficient to allow the lamp to reach its operating temperature, which is inconvenient. The delays can be eliminated through use of xenon lamps because they reach their operating temperature quickly. However, xenon lamps are expensive. Moreover, delaying the scanning process and/or using xenon lamps does not address the issue of light intensity variations over the life of the lamp.
More recently, optical scanners have been introduced that monitor the intensity of the lamp during each scan and automatically compensate for any variations in lamp intensity. Here, the mirror and lens arrangement focuses light that is reflected from a known reference (or “lamp monitor element”), such as a white patch located at one of the lateral edges of the object, onto a predetermined number of sensing elements of the CCD during the entire document scan. A light intensity compensation signal is generated for each pixel row based on the difference, if any, between the actual voltages generated by the sensing elements and the expected voltage. The compensation signal is used by the controller to adjust the signals where necessary. One example of a scanner that incorporates such a reference arrangement is disclosed in commonly assigned U.S. Pat. No. 5,278,674, which is incorporated herein by reference.
Although conventional scanners with lamp intensity monitoring capabilities are useful, the inventor herein has determined that these capabilities have not been fully implemented in multiple resolution scanners. The inventor has further determined that the failure to heretofore combine lamp intensity monitoring and multiple resolution scanning has been due to the fact that the conventional lamp monitor element, which is at the lateral edge of the object during a normal resolution scan, is not included in a higher resolution scan. Moreover, simply moving the lamp monitor element inwardly to a position where it will be at the lateral edge of the object in a higher resolution scan (or adding a second lamp monitor element at this position) will interfere with a normal resolution scan. Thus, conventional multiple resolution scanners could only monitor lamp intensity in the normal resolution mode and required users to wait for the lamp to reach its operating temperature before scanning in the higher resolution mode(s).